The invention relates to a method of determining position data of a measurement point and a device for measuring the magnification in an optical beam path, in particular in a beam path of a surgical microscope.
Surgical microscopes are used by a surgeon for the optical magnification of the area in which an operation is intended to be carried out. There are in principle three different types of surgical microscopes, all of which are meant in the sense of the invention. These are,
firstly
pure optical microscopes, that is to say microscopes which contain only optical and mechanical components, the output being directed to the eye; PA1 pure video microscopes, that is to say microscopes which have optical, mechanical and optoelectronic components, the optical output of the microscope being directed exclusively to an optoelectronic image recording device (for example a CCD), and the image recorded being further processed exclusively electronically and, if appropriate, being displayed via a display; and PA1 mixed video microscopes, which contain constructional features of the microscopes of the first and second type in common, that is to say that an output is directed both to an observer's eye and to an image recording device.
secondly
thirdly
As a result of the magnification of the area to be operated on, a surgeon loses the direct estimation of size which he/she has in the case of operations with the unaided eye. This leads to problems primarily where specific, previously determined cut depths or cut lengths are to be observed, or where the surgeon has to keep to specific distances using a surgical tool in order to make a precise operation possible. Above all in the case of operations on the brain or in microsurgery, this is often imperative in order to avoid damage to healthy tissue. In the case of such operations, the result of the operation (whether complete success or death) often depends on fractions of millimeters. Therefore, efforts have been made to determine the areas as precisely as possible and to permit measurements of sizes. As an example of such a known construction, reference is made to the German Patent Application DE-A-4134481.
In the DE-A mentioned, a surgical microscope is described in which an exact location determination is intended to be carried out of a specific point, generated by means of a laser beam, on an object being observed. For this purpose, a sighting method is proposed in which, by means of focusing and defocusing the microscope, respectively, visual field markings are brought into coincidence. After this, the exact determination of the position of the point on the object is intended to be possible, in that optical system data are used for calculation. These system data are intended to be determined, according to the DE-A, by means of suitable distance detectors or angle detectors on drive units for the respective positioning of positionable optical components. It is specifically intended to draw conclusions therefrom about the magnification of the magnification system.
The determination of the magnification is therefore carried out indirectly via the measurement of distances, angles or via sensors which are connected to positioning devices for optical components, and via a subsequent calculation of the corresponding data.
This is in many cases unsatisfactory and insufficient. The main reason lies in the fact that both the optomechanical components and the mechanical/electrical components (sensors) have tolerances which, under certain circumstances, change nonlinearly. This results in the risk that magnification values determined in this way are wrong and therefore the position data further determined therefrom are not correct. In the extreme case, such incorrect data could lead to serious errors during the work of the surgeon. Such errors are possibly somewhat lessened by--necessarily provided in accordance with the DE-A--calibration measurements on the patient. However, even these are not indisputable and depend primarily on the human capability of the operator. The known attempt to register mechanical tolerances of the magnification system during the assembly of the microscope and to determine therefrom a correction curve, which is superimposed onto the current data, is insufficient to the extent that tolerances may change as a function of countless factors, and the correction curves then used are of no help. In addition, the determination of such correction curves is itself problematic, above all time-consuming. A corresponding correction program, furthermore, requires additional computing power and, in some cases, reduces the computer speed in the real time area.